Batteries are charged by power obtained by utilizing renewable energy. Patent Literature 1 below describes a technique of charging a power storage unit with power generated by a solar power generator or a wind power generator.
In these days, there is an increasing demand for electricity in developing countries. Particularly, in developing countries, mobile phones and smartphones have been widely spread and more wireless base stations for mobile communications are also increasingly installed, so that the demand for electricity to be supplied to wireless base stations for mobile communications is markedly increasing. In such developing countries where the demand for electricity is markedly increasing, not only power generated by existing power plants but also the demand for electricity obtained by utilizing the renewable energy as described above is considered to increase.
In developed countries such as Japan, the power grid is well established and the power supply is stable, and thus an uninterruptible power supply (UPS) is mainly used in general as a backup power supply during power outage. Even when the power outage occurs, the power is recovered in a short time in developed countries, so that a backup power supply using a small capacity uninterruptible power supply is efficient.
However, in developing countries where the power grid is not well established and the power supply is not stable, once power outage occurs, time of the power outage extends over a long period of time and backup, such as a small capacity uninterruptible power supply, is not effective. Even in developed countries, there is a case that power outage over a long period of time has to occur at the time of disaster and, including the case of developing countries described above, the importance of backup power supply is enhanced.